1. Field of the Invention
The present invention relates to a switching control circuit and its control method for controlling between the on state and the off state of a power device.
2. Description of Related Art
A device such as an inverter usually has a plurality of power devices and a switching control circuit that controls the on/off state of the power devices. In some cases, this type of switching control circuit includes a protection circuit that protects the device against a short circuit current that flows when a short circuit occurs between the upper arm and the lower arm. For example, when an open fault occurs in the power device of one of the upper arm and the lower arm with the result that the arms are short-circuited, the protection circuit detects a short circuit current flowing in the power device that has no open fault and turns off the power device. The inverter protection circuit disclosed in Japanese Patent Application Publication No. 7-255182 (JP 7-255182) is configured such that, when a short circuit is detected in one of the arms, the protection circuit slowly decreases the gate potential of the power device of the other arm. Decreasing the gate potential in this manner slowly turns off the current flowing in the power device, thus preventing the power device from being destroyed by a surge voltage.
The protection circuit of a switching control circuit sometimes disables its function for a predetermined period (mask period) immediately after the power device is turned on. Disabling the protection function in this way prevents the protection circuit from detecting an inrush current, which flows at turn-on time, as a short circuit and from turning off the power device. However, if a short circuit actually occurs in the power device of one of the arms during the mask period in this switching control circuit, the short circuit current continues to flow into the power device of the other arm. As a result, when a short circuit occurs in the power device of one of the arms during the mask period, a large amount of energy is accumulated in the power device of the other arm.
The inverter protection circuit, disclosed in Japanese Patent Application Publication No. 7-255182 (JP 7-255182), slowly turns off a power device and, therefore, generates a large off-state power loss with the result that a large amount of energy is applied to the power device at turn-off time. Therefore, when a mask period is provided in the protection circuit and a short circuit occurs during that mask period, a large amount of energy, which is applied at turn-off time, is further added to the power device in which the energy is already accumulated during the mask period. As a result, the total amount of applied energy exceeds the energy tolerance limit of the power device, sometimes leading to the destruction of the power device.